Have you tried measuring the voltages on the IN1 and IN2 pins when you are trying to run the motor in each direction?
If you swap the connections to IN1 and IN2, is the "bad" button still the same one?
nope that's one of the first things I did, and I even swapped the buttons. The motor runs nicely in rev with IN1 & IN2 swapped.
now here's another question do these/other H-Bridges normally have a voltage drop like that? I'm expecting 12v or slightly lower output with a 12v supply... or have I got it wrong? The datasheet mentions 1ohm 'on' resistance. I'm assuming that wouldn't yank it down to 7.8V like that...
the other thing is how sensitive are the arduinos to shorts? because I have messed up on a few occasions where the arduino would rest itself could I have wrecked my arduino, or both of them?